pool: error quickly if we find a port in use

Before the fix, the added test would give a Pool.Allocate error like:

	pool could not connect to 9000: timeout waiting for initial target

The actual underlying error, which can only be seen if one inspects
chrome's stderr, is that it failed to bind to the debugging protocol
port if it was already in use.

This is of course an issue with the environment that chromedp is being
run under, since it was given a port range that wasn't available.
However, the confusing error can lead to developers wasting their time
instead of spotting the error quickly.

Unfortunately, there doesn't seem to be a way to have Chrome exit
immediately if it can't bind to the given port. So, instead of relying
on it, check if the current process can bind to the port first.

Add a test too, where we grab the first port in the pool range, and
check that we get an error that's not confusing.

Fixes #253.

pool.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"net"
 	"sync"
 
 	"github.com/chromedp/chromedp/runner"
@@ -70,6 +71,18 @@ func (p *Pool) Allocate(ctxt context.Context, opts ...runner.CommandLineOption)
 
 	r := p.next(ctxt)
 
+	// Check if the port is available first. If it's not, Chrome will print
+	// an "address already in use" error, but it will otherwise keep
+	// running. This can lead to Allocate succeeding, while the chrome
+	// process isn't actually listening on the port we need.
+	l, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", r.port))
+	if err != nil {
+		// we can't use this port, e.g. address already in use
+		p.errf("pool could not allocate runner on port %d: %v", r.port, err)
+		return nil, err
+	}
+	l.Close()
+
 	p.debugf("pool allocating %d", r.port)
 
 	// create runner

pool_test.go

@@ -0,0 +1,43 @@
+package chromedp
+
+import (
+	"context"
+	"net"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestAllocatePortInUse(t *testing.T) {
+	t.Parallel()
+
+	// take a random available port
+	l, err := net.Listen("tcp4", "localhost:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer l.Close()
+
+	ctxt, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// make the pool use the port already in use via a port range
+	_, portStr, _ := net.SplitHostPort(l.Addr().String())
+	port, _ := strconv.Atoi(portStr)
+	pool, err := NewPool(PortRange(port, port+1))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	c, err := pool.Allocate(ctxt)
+	if err != nil {
+		want := "address already in use"
+		got := err.Error()
+		if !strings.Contains(got, want) {
+			t.Fatalf("wanted error to contain %q, but got %q", want, got)
+		}
+	} else {
+		t.Fatal("wanted Allocate to error if port is in use")
+		c.Release()
+	}
+}